The present invention relates to an airfoil for use in a turbo-machine. More specifically, the present invention relates to an airfoil for use in a stationary vane in the turbine section of a gas turbine.
A gas turbine employs a plurality of stationary vanes that are circumferentially arranged in rows in the turbine section of the gas turbine. Such turbine vanes are exposed to the hot, sometimes corrosive and particle laden, gas discharging from the combustion section. This exposure creates a variety of problems in the vanes, including corrosion, erosion, and cracking due to over-temperature and thermal stress. Such problems can reduce the useful life of the vane, thereby increasing the operating cost and reducing the availability of the gas turbine.
The corrosion, erosion and cracking problems are often most severe at the leading edge portion of the vane airfoil because the hot gas flows directly into the leading edge and creates high heat transfer coefficients. Unfortunately, since vane airfoils have generally employed an integral, cast construction, repair of the leading edge portion required a difficult and expensive welding operation.
One approach to improving vane life that has been attempted in the past is to form cooling air passages in the leading edge portion of the airfoil so as to reduce its temperature. Unfortunately, the use of such cooling air, which bypasses the combustion section, detracts from the thermodynamic performance of the turbine. Another approach used in the past involves forming the vanes from ceramic materials, which have excellent high temperature strength, as well as corrosion and erosion resistance. Unfortunately, such ceramic materials are brittle and unable to withstand the high thermal stresses imposed by the differential thermal expansion that occurs among the various portions of the vane, especially during start-up and shutdown.
It is therefore desirable to provide a gas turbine vane having a leading edge portion that can be readily repaired and that can be made from a ceramic material without imposing high stresses due to differential thermal expansion.